1. Field of the Invention
The present invention relates to the field of marine seismic cables. More specifically, the present invention relates to a solid marine seismic cable with an array of hydrophones.
2. Description of the Related Art
As the name suggests, marine seismic cables are used to measure underwater sound. Marine seismic exploration and data acquisition is typically conducted using a vessel towing one or more cables with a plurality of measuring devices. An acoustic source is provided in the vicinity of the seismic cable, by an air-gun or various other means. The source directs acoustic energy into the earth, and some of the energy is reflected back to the surface from the different layers of rock below the surface. The returning waves are detected with sensitive measuring devices and the system accurately records the wave and the time it has taken to travel through the various layers in the earth and back to the surface. A number of processing steps may be made on these recordings such that they are transformed into visual images which give a picture of the subsurface of the earth beneath the seismic survey area. Marine seismic surveys can be conducted for a variety of reasons, including oil and gas exploration, site characterization, or other environmental investigations.
One type of measuring device that is commonly used in marine seismic surveys is a marine seismic cable. Such cables, or at least sections thereof, may contain arrays of hydrophones. A hydrophone is essentially a microphone designed to be utilized underwater. The hydrophones are assembled together along with other associated equipment and towed behind a seismic survey vessel when performing a seismic survey.
A typical cable section is 25 to 150 m long, while an assembled streamer section comprising several cable sections may be as long as 10-15 km. The hydrophones are distributed along the length of the cable, which may include spacers, strength member(s), and flotation elements, in order to render the cable section neutrally buoyant. The cable section also includes a means of electrical connection to the various hydrophones, which may be connected in various ways to form serial or parallel groups. The cable section may also contain electronics for pre-amplification and/or digitization of the acoustic signals measured by the hydrophone group(s).
A variety of methods have been used to construct such cables. In liquid-filled cables, the buoyancy is adjusted by filling the space inside an outer plastic sheath with a non-conductive fluid such as kerosene. Such cables are known to suffer from certain specific types of acoustic noise which may be transmitted through the fluid to the hydrophones (“bulge” waves). Liquid filled cables may also be easily damaged during normal operation. Also, the fluids used as fillers may be environmentally sensitive or flammable. Thus, various methods have been proposed to build solid or semi-solid streamers, which are inherently less susceptible to certain acoustic noise sources and are more ruggedly constructed to allow ease of use, handling, and deployment.
Replacing the fluid in the cable with a gel has been proposed. Gel filled streamers may still be sensitive to certain noise sources from within or outside the outer plastic sheath, and are difficult to build. Solid streamers, though more durable and substantially immune to bulge wave noise, are stiff and unwieldy during deployment. Also, such solid cables often include some means of incorporating the hydrophone array into the resulting structure using liquid-filled pockets. In summary, there has not been available a fully solid seismic cable section having the desired properties of superior acoustic performance, small diameter, rugged construction, and long-term stability, with the required flexibility and ease of use.